It has long been known in the art that the shelf life of a food may be extended by subjecting the food to heat treatment which is so thorough that bacteria, fungi and other micro-organisms present in the food are killed-off, neutralised or reduced in number. In order to achieve complete sterilization, the treatment must be carried out under such forms that it is certain that all parts of the treated product are heated to such an elevated degree and for such a length of time as are required for complete destruction or deactivation of the micro-organisms in the product.
Different types of food products require different forms of treatment. For example, milk or lactic products must be subjected to more extensive treatment (higher temperature and/or longer treatment time) than, for example, acidic products such as fruit juices of different types. The reason for this is that the basic preconditions for growth of the micro-organisms in an acidic environment are substantially poorer than if the environment is basic or neutral. By way of example, mention might be made of the fact that milk can be sterilized by means of a short heating time up to approximately 140.degree. C., for a few (4 to 10) seconds, while on the other hand orange juice can be treated to complete sterilization at a temperature of approximately 100.degree. C. and less, in a treatment time of a few seconds or a few tenths of a second. Since extensive heating of food products generally gives rise to a deterioration in flavour and also a deterioration in the quality of the finished product, attempts are made in the art to restrict the effects of the heat to as little as possible, which is generally achieved by heating to a relatively high temperature for a short time.
In that case when the product which is to be heat-treated is aqueous (as is the case in milk and juice), heating to a temperature above 100.degree. C. cannot take place without the water in the product being caused to boil, unless the treatment takes place in an enclosed space in which an excess pressure can be maintained and the boiling point of the product can thereby be raised. Such an apparatus goes under the general title of an autoclave, and an autoclave is thus an apparatus which comprises an enclosed treatment chamber in which both temperature and pressure may be controlled and regulated. One of the drawbacks inherent in autoclaving is that the treatment is largely carried out intermittently for a number of objects inserted in the autoclave (so-called batch treatment), and that the treatment time depends on the length of time that elapses to ensure total treatment, i.e. by heating of the product placed in the autoclave.
Since heat is to be supplied to the product through its enclosing or packaging material, and be led in to the central regions of the product, it is advantageous to keep the thermal transfer path to the central areas of the product as short as possible in order thereby to limit the total heating time of the product. The product must, thus, be as thin or flat as possible during the heat treatment, i.e. have a least possible extent in one dimension, but this requirement on the product and its packaging form generally runs counter to other requirements placed on the packaging container, for example that it be of a configuration that makes it easy to distribute and store, and that it must be able to stand upright on its own on a flat substrate without tipping over.
In the present case, this problem is solved in the manner apparent from the appended claims.